Since the planet receives a net of about 100 W/m2, would a change of 4 W/m2 be considered substantial? Explain. (Chapter 17 shows that this is the size of the greenhouse effect.)
What will be an ideal response?
Yes, a 4% change is substantial on this scale. The Little Ice Age was due to a
change of about 1/300 = 0.3% in temperature.
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This graph shows the frequency of impacts on Earth by objects of various sizes. According to this graph, objects large enough to cause a mass extinction hit Earth
A) about once every 50 million years. B) about once every 1,000 years. C) about once in Earth's history. D) about once every 500 million years.
A 3 kg mass is positioned at (0, 8), and a 1 kg mass is positioned at (12, 0). What are the coordinates of a 4 kg mass which will result in the center of mass of the system of three masses being located at the origin (0, 0)?
A) (-3, -6) B) (-6, -3) C) (3, 6) D) (-12, -8)
To reduce the home heating requirements, modern building codes in many parts of the country require the use of double-glazed or double-pane windows, i.e., windows with two panes of glass. Some of these so called thermopane windows have an evacuated space between the two glass panes while others trap stagnant air in the space. (a) Consider a double-pane window with the dimensions shown in the sketch given below. If this window has stagnant air trapped between the two panes and the convective heat transfer coefficients on the inside and outside surfaces are 4 W/(m2 K) and 15 W/(m2 K), respectively, calculate the overall heat transfer coefficient for the system. (b) If the inside air temperature is 22°C and the outside air temperature is –5°C, compare the heat loss through a 4 m2
double-pane window with the heat loss through a single-pane window. Comment on the effect of the window frame on this result. (c) If the total window area of a home heated by electric resistance heaters at a cost of $.10/kWh is 80 m2 how much more cost can you justify for the double-pane windows if the average temperature difference during the six winter months when heating is required is about 15°C?
GIVEN
- Double-pane window with stagnant air in gap
- Convective heat transfer coefficients
- Air temperatures
FIND
(a) The overall heat transfer coefficient
(b) Compare heat loss of double- and single-pane window
(c) Cost to justify for the use of double-pane window.
ASSUMPTIONS
- Steady state conditions prevail
- Radiative heat transfer is negligible
SKETCH
The full moon by definition occurs
a. for only a day. b. for 7-3/8 days. c. only for an instant. d. only once every month.